The technical field of this invention is that of nondestructive materials characterization, particularly as it applies to postweld and in-process weld scanning for quality control, in-process monitoring, and seam tracking using spatially periodic field eddy current sensors.
There is an increasing need for a nondestructive method for assessing the quality of welds between materials, including the detection and characterization of defects. In particular, friction stir welding is becoming more commonly used as a joining technique for a variety of metals, including aluminum, titanium and nickel base alloys as well as steels. The quality of the weld depends upon a variety of factors, including the materials, the rotation rate, feed, positioning, applied pressure from the pin tool, and the penetration ligament. Defects such as cracks, lack of penetration (LOP), and lack of fusion can compromise the integrity of the joint and can lead to component failure.
Weld examinations are currently performed to characterize quality of the welds, qualify a welding procedure or qualify welders. These examinations are performed to detect cracks, lack of fusion, lack of penetration, areas of excessive porosity, or unacceptably large inclusions. Liquid penetrant inspection (LPI) is widely used for detection of surface-connected defects in welded components fabricated from nonmagnetizable materials. In some cases, LPI fails to detect these surface-connected defects, such as in the case of tight cracks, cracks densely filled with foreign matter, or weakly-bonded LOP defects in friction stir welds (FSWs).
For components fabricated from magnetizable materials, such as carbon and low-alloy steels, magnetic particle inspection (MPI) is typically used for detection of surface-connected cracks. Some MPI techniques are claimed to detect cracks that are masked by smeared metal so that the cracks are not directly exposed to the surface. Furthermore, MPI is permitted for inspection through thin coatings typically less than 0.003 in. (0.075 mm) thick. However, MPI is limited in crack detection capability and, for coated surfaces, may require coating removal. Methods are needed to inspect carbon and low-alloy steel components for cracks that are below the MPI detection threshold and for inspections that do not require coating removal. There is also a need to characterize residual stresses in these welds. Other conventional nondestructive testing methods such as conventional eddy current sensing are limited in their sensitivity to small flaws in welds and in their capability to extract spatial information about changes in the weld microstructure and flaw characteristics. The use of conventional eddy current sensing often involves extensive scanning along and across the weld.
Etching with a variety of metallographic etchants is also used to reveal macrostructural or microstructural characteristics of welded joints, including weld metal, heat-affected zone, and base metal. In the case of FSW, which is joining by plastic deformation and stirring below solidus, etching can reveal the dynamically recrystallized zone (DXZ), thermomechanically affected zone (TMZ), heat-affected zone (HAZ) and base metal. Etching of FSWs can also be used as a method for characterizing LOP defects, by revealing the relevant width of the DXZ. For example, as shown in FIG. 1, the DXZ, TMZ and HAZ show up after etching as distinctly different zones permitting direct measurement of the width of the DXZ that has penetrated to the backside of the welded panels. Etching of panels joined by FSW would, in the case of butt welds, reveal these zones on both the front and back sides. Unfortunately, the etching process is time consuming, not practical for inspection of long welds required for large structures, such as spacecraft and aircraft, not environmentally friendly, and often not permitted in production. Methods are needed to inspect these surfaces rapidly and nondestructively.
It is often critical to characterize microstructural variations of metal products such as ingots, castings, forgings, rolled products, drawn products, extruded products, etc. Etching of selected samples is used for this purpose but is not practical or permissible for large surfaces or statistically significant quantities, areas, or lengths. It is definitely not acceptable for 100 percent inspection of these products when information on microstructural variations, including imaging of these variations and their quantitative characterization, is required over the entire surface of a product. Furthermore, etching of large surfaces in components that are suspected to contain local zones that are different due to fabrication problems, service-induced or accident-induced effects is not practical, unless the locations of such zones are known a priori.
The use of eddy current sensors and high resolution conformable eddy current sensor arrays permits quality control monitoring for fusion welds, friction stir welds (FSWs), metal products such as ingots, castings, forgings, rolled products, drawn products, extruded products, etc., and components with locally different microstructures. In one embodiment, the quality of the joint or weld is determined from eddy current measurements of the test material properties across the weld region by determining a feature of the weld from a combination of the electrical property measurement and the location information. In an embodiment, the electrical property of the test material used to determine the feature is the electrical conductivity. In one embodiment, the feature is the width of the dynamically recrystrallized zone (DXZ). Descriptions for FSWs may also be applied to other weld methods.
In another embodiment, friction stir welds are characterized by eddy current sensors and sensor arrays having a meandering drive winding with extended portions for imposing a magnetic field. In another embodiment, the drive winding forms a modified meandering pattern that approximates a periodic field as described in patent application No. 60/276,997, filed Mar. 19, 2001, the entire teachings of which are incorporated herein by reference. The windings can be fabricated onto rigid or conformable substrates. Sensing elements placed between the extended portions of the drive winding respond to the properties of the test material. A single sensing element can be placed between each pair of extended portions and electrically connected to each other sensing element to provide a single output response for the sensing when scanned over the test material. Alternatively, numerous sensing elements can be placed in rows parallel to the extended portions. This facilitates imaging of the material properties, particularly when the sensor array is scanned in a direction perpendicular to the row of sensing elements. In one embodiment, the sensing elements are coils that couple to the drive windings through induction and the sensing windings have dimensions small enough to provide imaging resolution suitable for measuring the width of the weld region at or near the surface, e.g., at the crown or root of a fusion weld or DXZ that penetrates through the plates joined by FSW. In a second embodiment, the sensing elements incorporate magnetoresistive sensors to permit inspection down to low frequencies (such as a 50 Hz or even dc) for characterization of relatively thick plates, such as 0.5 in. (12.5 mm) aluminum lithium alloy plates. In one embodiment, the sensor construct uses a circular or rectangular distributed drive winding that excites a smoothly varying shaped magnetic field. In a particular embodiment, the magnetoresistive elements are giant magnetoresistive sensors.
Scanning of the sensors over the weld region permits the quality of the weld to be determined through features of the electrical property profile across the weld. The orientation of the sensor, relative to the weld axis, can be varied to adjust the sensitivity to the different types of defects, such as intermittent planar flaws, lack of penetration (LOP) of the tool tip, and weak metallurgical bonds. When deep penetration is used, other defects such as porosity, internal flaws, cracks, and weak bonds are imaged or detected. This can apply to butt joints, lap joints, or other weld geometries. In one embodiment, the extended portions of the sensor are oriented parallel to the weld axis. In another, the extended portions are oriented perpendicular to the weld axis. With each orientation, the sensor can be scanned across the weld, perpendicular to the weld axis, or along the weld, parallel to the weld axis. Scanning the sensor along a path that forms a small angle, such as 15 degree, with the weld axis, with the extended portions oriented perpendicular to the translation path, provides measurement sensitivity to both longitudinal and transverse flaws.
For the features used in determining the weld quality, in one embodiment the electrical property is the electrical conductivity. In another embodiment, the electrical property is the magnetic permeability. In another embodiment, the feature is the width of the weld at different depths determined using multiple frequency measurements. The weld quality could then be indicated by the LOP thickness or the presence of planar flaws. In another embodiment, the weld quality feature is obtained from images of the electrical property variations over the region of the weld. Again, in this case, the quality of the weld can be indicated by the presence of planar flaws, weak bonds, or other defects.
The frequency of the excitation also influences the measurement response and can be used to determine the quality of the weld. In one embodiment, a single high frequency measurement is made of conductivity and proximity at each sensing element to measure only the near surface properties of the material. In another embodiment, multiple frequencies are used to determine the variation of material properties with depth from the surface. This includes the generation of three-dimensional images of FSW, including the weld nugget or DXZ, using model based methods that model the magnetic field interactions with the nugget; these methods can be either analytical or numerical, such as finite element methods. In one embodiment, the model is used to generate two-dimensional measurement grids and higher-order multi-dimensional databases, respectively, of sensor responses to FSW zones, including the DXZ, property variations. In one example, the estimated properties of the DXZ are the width of the penetration zone at the base of the weld and the width of the DXZ at a selected depth from the base of the weld. In another example, the material properties are the conductivity of the LOP region and the thickness of the LOP defect thickness. The multiple frequency imaging method is then used to estimate these two parameters using a combination of measurement grid table look-ups, intelligent root searching methods, or apriori knowledge of the nugget geometry to estimate nugget geometry parameters. The frequency can range from 100 Hz to 10 MHz. In another embodiment, dissimilar welds are inspected and the shape of the electrical conductivity response determines the weld quality. A good weld has a gradual transition while a bad weld has a more abrupt transition between the plates for a butt weld.
In another embodiment, a sensor array is used to characterize subsurface features such as porosity, cracks, lack of fusion, material condition and properties before and after heat treatment (or other processes), as well as other material anomalies or property distributions that affect metal product, component, or weld quality. In another embodiment high frequencies (100 kHz to 10 MHz) are used to detect surface breaking flaws as an automated replacement for liquid penetrant testing.
In another embodiment, the sensing elements include magnetoresistive sensors. Similar to the inductive coils, images of the material properties can be obtained by scanning rows of magnetoresisitive elements oriented parallel to the extended portions of the drive winding. This image can be formed from the electrical property measurements across and along the weld region. In an embodiment, the weld quality is indicated by the surface and through thickness properties of the weld region. The weld quality can be indicated by the presence of a crack-like defect, an LOP defect, the presence of an internal flaw, or a weak metallurgical bond. In another embodiment, an LOP defect can be detected by scanning the sensor over the top surface such that the LOP defect is on the opposite side of the weld. The magnetoresistive sensing elements may further comprise encircling secondary coils to improve the dynamic range of the measurements and bias the magnetoresistive sensors, as described in patent application Ser. No. 10/045,650 filed Nov. 8, 2001, entititled xe2x80x9cDeep Penetration Magnetoquasistatic Arrays,xe2x80x9d by Sheiretov et al., Attorney Docket No. 1884.2007-001, the entire teachings of which are incorporated herein by reference. This provides a potential replacement for radiography or phased array ultrasonics for thick plate (0.25 to 1 inch thick) inspections. The secondary coils can be used in a feedback configuration with external electronic circuitry to maintain the field in the vicinity of the magnetoresistive element.
For magnetizable metal products, components, and welds, such as carbon and high-strength low-alloy steels, the magnetoresistive sensing element arrays are used to measure from DC to high frequencies and map residual stress patterns and the geometry of the weld regions. In one embodiment, scans are made with both inductive sensing elements and magnetoresistive sensing elements to provide inspections from DC up to high frequencies (such as 10 MHz). For these materials, the high resolution imaging with conformable eddy current sensor arrays that use a single wavelength drive winding with an array of sensing elements is a direct replacement for magnetic particle inspection and does not require paint removal. In another embodiment, multiple frequencies are used to measure the depth of cracks that are either surface breaking of subsurface. In one embodiment the bi-directional permeability is related to weld residual stress and heat affected zone residual stresses.
In an embodiment, the eddy current sensors and eddy current sensor arrays having drive windings with extended portions can also be used for the quality control of joining processes. In one embodiment, the joining process involves tracking, such as locating or following, the seam between the joined materials. Furthermore, varying the orientation of the extended portions with respect to the seam axis also provides information about the seam orientation. In an embodiment, the electrical property of the measurement is the electrical conductivity. In another embodiment, the joining process is a friction stir welding process. One embodiment further comprises mounting the sensor in the anvil to monitor the weld process beneath the welding tool. Another embodiment comprises mounting sensors ahead of and behind the anvil on the opposite side as the weld. Another embodiment comprises mounting sensors ahead of and behind the welding tool on the same side as the weld.
In another embodiment, a sensor is used to control the tool and the position of the sensor with respect to the tool position is kept constant. This configuration can be applied to a fixed material with the tool moving or a fixed tool position with the material moved past the tool. In one embodiment, a sensor is placed over the front surface of the material. In another embodiment, another sensor is placed behind the test material for monitoring the weld processes on the surface opposite the weld tool. In each of these cases, a preferred embodiment has the sensors not in contact with the test material.